The Talking Machine Forum — For All Antique Phonographs & Recordings

I have a question concerning how mica diaphragms are made. Does anyone know the process how they were made? How they are made today? I know that mica starts out as a mineral in the ground. How does it made from a raw material to a finished product?
Harvey Kravitz

Thanks for asking, Harvey...
I now know more about mica than I ever cared to learn and will share some of it with you. Mica is a mineral, which is found in various parts of the world. In the US, it is found in Spruce Pine, NC and Virginia. Worldwide, it is found in India, Russia and China, with probably the largest production coming from India. It is a unique form of rock which can be separated into thin layers and is useful in various industries.

Sheet mica is used in electrical components, electronics, isinglass, and atomic force microscopy. Other uses include diaphragms for oxygen-breathing equipment, marker dials for navigation compasses, optical filters, pyrometers, thermal regulators, stove and kerosene heater windows, radiation aperture covers for microwave ovens, and micathermic heater elements. Mica is birefringent and is therefore commonly used to make quarter and half wave plates. Specialized applications for sheet mica are found in aerospace components in air-, ground-, and sea-launched missile systems, laser devices, medical electronics and radar systems.

Sheet mica is used principally in the electronic and electrical industries. Its usefulness in these applications is derived from its unique electrical and thermal insulating properties and its mechanical properties, which allow it to be cut, punched, stamped, and machined to close tolerances.

Because of these properties, phonograph diaphragms are punched out of sheets of mica and you can make your own by cutting with metal shears or tool type scissors (don't use your wife's dressmaking scissors). Most likely, phonograph diaphragms are made from sheets of isinglass, which is used for windows in wood burning stoves and is a lower grade of mica. High quality mica is expensive and therefore prohibitive to use.

Isinglass
Thin transparent sheets of mica called "isinglass" were used for peepholes in boilers, lanterns, stoves, and kerosene heaters because they were less likely to shatter than glass when exposed to extreme temperature gradients. Such peepholes were also used in "isinglass curtains" in horse-drawn carriages and early 20th century cars and phonograph diaphragms.

How to make your own diaphragms:
https://www.youtube.com/watch?v=1aGzz_G07KY

Suppliers:
http://stovemicasupply.com/
http://www.ashevillemica.com/
Asheville-Schoonmaker Mica Co. and Reliance Mica

As always... thanks for asking...

Wow, Curt. That was very informative!
I'm glad Harvey asked that question.

Thanks, James

What thickness is recommended for reproducers?

Knew about all the electronic uses so it's great to know where else Mica gets used...thank for the information you posted....

Curt, Thanks for the informative answer. How is the mica cut into the thin sheets of isinglass? Starting from a raw mineral, it must be difficult and tedious work.
Harvey Kravitz